There is much interest at the present time in fabricating photodetectors useful in detecting radiation having wavelengths longer than 1.0 .mu.m. Such photodetectors would be useful in, for example, optical communications systems operating in the wavelength region between 1.3 and 1.6 .mu.m which is the region that includes the wavelengths of both minimum loss and minimum dispersion for silica based optical fibers.
Avalanche photodetectors are desirable for many purposes because of the gain they provide within the detector. A variety of semiconductor materials has been used in such photodetectors which should have desirable characteristics such as low noise. Silicon is an almost ideal semiconductor with respect to its noise performance because of the large ratio of ionization coefficients, but it cannot be used for wavelengths longer than 1.0 .mu.m because its bandgap is too large. The large ratio of ionization coefficients leads to a low excess noise factor. Germanium has a bandgap which would permit it to absorb radiation at wavelengths as long as 1.6 .mu.m, but it is not an ideal avalanche photodetector material because its ratio of the ionization coefficients is approximately one and a large excess noise factor results. Perhaps even more significantly, it has a relatively small bandgap and there is a large dark current at room temperature.
Of course, one might think that the problems that are apparently inherent in both germanium and silicon might be circumvented by absorbing the radiation in a germanium region and letting the avalanche process initiated by the photogenerated carriers occur in a silicon region. Such an approach using separate absorption and multiplication regions has been proposed. See, for example, U.S. Pat. No. 4,212,019 issued on July 8, 1982, Wataze et al (Wataze) and especially his Examples 1 and 3. These Examples describe a silicon avalanche multiplication region and Ge and Ge.sub.x Si.sub.1-x, respectively, absorption regions. This approach is, however, not free of difficulties. For example, as is well known, there is a relatively large lattice mismatch between silicon and germanium. Therefore, simply growing germanium on a silicon substrate or epitaxial layers will yield high quality, i.e., defect free, germanium layers only if the germanium layers are relatively thin. This is practically impossible as the germanium layers should be less than 10 Angstroms thick. However, this is undesirable for avalanche photodetectors because germanium-silicon alloys and elemental germanium, being indirect bandgap materials, have relatively low absorption coefficients in the wavelength region of interest for optical communications. Thus, a photodetector having high quantum efficiency, i.e., high absorption, would require a relatively long optical path length which is difficult, if not impossible, to achieve in conventional photodetector designs using Ge and Si. In such designs the photogenerated carriers move either substantially parallel or antiparallel to the direction of light propagation.
Growing the alloy layer with only a small amount of Ge may alleviate problems caused by the mismatch but will not permit absorption at long wavelengths. This problem increases as the wavelength of the incident radiation becomes longer because the bandgap decreases as the germanium fraction increases. It is especially severe when optical communications applications near 1.55 .mu.m are contemplated. To obtain a bandgap low enough to enable the germanium silicon alloy to absorb at 1.55 .mu.m, the germanium fraction in the alloy layer must be very large. However, it is difficult to grow these alloy layers with both an appreciable thickness and a large germanium fraction without generating high dislocation densities. Thus, absorption will be undesirably low at 1.55 .mu.m due to the thinness of the layer. Essentially, identical considerations are applicable at 1.3 .mu.m.
Although it has been known for a long time that several effects, including pressure, can alter the bandgap, it has generally been believed by those skilled in the art that the effects arising from any attainable change in the pressure would be too small to alter the bandgap by a significant amount with respect to parameters considered for device design. For example, it was believed that it would be too small to alter the bandgap of a germanium silicon alloy sufficiently to make such alloys useful for use in photodetectors at 1.55 .mu.m. Of course, it was also generally believed that the effect of strain induced bandgap variations would be too small to lead to useful effects in other types of devices. For example, light emitting devices might emit at wavelengths other than expected from the properties of the bulk materials but the expected shift would be small.